She already had cracked her racket, lost 12 consecutive points -- and the first set -- to Stosur when the anger and frustration that slowly had been building were suddenly loosed. Trouble was, she had just stroked a vicious forehand that was going to win her the point, but she bellowed "Come on!" before the ball reached Stosur.
That is a classic violation of tennis' intentional hindrance rule, the equivalent of unsportsmanlike conduct in this genteel sport. Chair umpire Eva Asderaki correctly awarded the point to Stosur, who merely got a racket on the ball. That gave her the first game of the second set, a service break that looked for all the world to be the end of Serena.
It wasn't. In fact, for a while, on this 10th anniversary of the Sept. 11 attacks, it looked like the beginning.
For a few games, Stosur looked overwhelmed. She was broken back immediately, and for a while, Williams seemed destined to win her 14th Grand Slam singles title.
But the muscular 27-year-old from Brisbane, Australia, playing the match of her life, found equilibrium. She rallied to beat Williams 6-2, 6-3, and the last point, a ripped forehand crosscourt winner, left Williams stumbling.
It was an upset of sweeping proportion, only the third title of Stosur's career.
"I'm still kind of speechless," Stosur said more than an hour later. "I can't believe I won this tournament. I did believe I had a chance to win. Two wins over her in the past made me think it was possible.
Said Serena, "Yeah, she played really, really well. It's good to see I tried my hardest, but she kept hitting winners.
Stosur had crossed the threshold of a major final only once -- the French Open last year -- and her failure there gripped her like a dogged virus for months. Fifteen months later, this had to feel really good.
It seemed appropriate that Stosur had to weather the longest match in U.S. Open history in terms of time (3 hours, 16 minutes) in the third round and the longest women's Grand Slam tiebreaker (17-15) to arrive at this place.
For two weeks, many had conceded the U.S. Open title to Williams, who had ripped though the field here without really being tested. Her victories over Victoria Azarenka and Caroline Wozniacki were, in some minds, the true finals.
Sunday, however, Serena did not look at all like herself. Stosur was faster, more fluid and far more forceful than Williams. Was it the painful tendon in her right foot? An empty tank after a long layoff and a late-Saturday night semifinal finish versus Wozniacki?
Serena, who famously threatened to stuff a ball down the throat of the lineswoman who called a foot fault that ended her semifinal match with Kim Clijsters, went fairly ballistic -- but stopped short of the expletives that surfaced two years ago. She was, after all, in the last match of a two-year probation.
"Aren't you the one who screwed me over last time here?" she asked Asderaki, perhaps confusing her with then-chair umpire Louise Engzell. "Do you have it out for me? That's totally not cool."
Later, as her tirade escalated, she defended her right to vent, saying, "We're in America last time I checked." In a final jab, she called Asderaki "unattractive inside."
Serena did not shake her hand after the match as is the custom.
"Everything happened so quickly out there," Stosur said. "The rules of tennis are there for a reason, and she made the call she felt was right. I can't even totally remember the point.
"For sure, it was difficult to stay focused. That was the loudest crowd I've ever heard. You're right in the middle of it, so it's quite an overwhelming feeling."
According to ITF rules, the chair umpire has the latitude not to make the call if he or she thinks the hindrance was unintentional.
Did Serena deliberately hinder Stosur?
"No," she said, laughing. "I didn't. I thought it was a clear winner.
Does she regret what she said?
"I regret losing, but there was nothing I could do," Serena said. "I don't even remember what I said. Sorry. I guess I'll see it on YouTube."
These women's finals at the U.S. Open have gone the two-set minimum for quite some time. The last one to go three sets, the maximum limit, was in 1995, when No. 1 seed Steffi Graf took down No. 2 Monica Seles. The year before, Arantxa Sanchez-Vicario beat Graf in three.
Who would have imagined it would be Stosur to extend the streak?
A dozen years ago, Williams won her first U.S. Open title in 1999, beating Martina Hingis in the final. It was the first of three titles for the 17-year-old.
Since then, there have been five other champions -- Kim Clijsters (three), Venus Williams and Justine Henin (two), Svetlana Kuznetsova and Maria Sharapova (one). Where are they now? Clijsters already has retired once and seems destined to step away for good after the 2012 Olympics to be a full-time mother. Venus, now 31, revealed here that she is struggling with an autoimmune disease that no doubt will compromise what is left of her tennis. Henin, after retiring twice, is a reality television star in Belgium. Kuznetsova and Sharapova are still playing tennis, but it is reasonable to assume they won't win another major.
Serena's lifestyle, focus and devotion to tennis have been widely questioned, but she is still standing on the baseline and hitting bombs. Despite the myriad physical setbacks, the yo-yoing in and out of the top 10, the parade of interesting friends and lovers, she continues not just to survive but also to thrive.
"I don't like second place," Serena said later. "But it's going to propel me to work harder."
Greg Garber is a senior writer for ESPN.com.